The present invention relates to gas sensors and more particularly to gas sensors employing a variable resistance gas sensing element.
Semiconductor gas sensors are known which change their resistance in response to the presence of gas molecules on their surfaces. A gas sensor such as, for example, a Taguchi Gas Sensor (TGS sensor hereinafter) manufactured by Figaro Engineering, Inc., Osaka, Japan, includes a semiconductor surface with an integral heater element to maintain appropriate operating conditions. The resistance between terminals of a TGS sensor is determined by the number of free carriers in the semiconductor material. When a gas molecule is absorbed on the surface of the semiconductor, it may bind a free carrier to reduce the conductivity thereof. When a TGS sensor is normally exposed to air, absorbed oxygen increases the resistance of the material to a substantially constant value. When a TGS sensor is exposed to other gases such as natural and other combustible gases, alcohol, ketone, ester and benzol families, as well as certain toxic gases, the resistance thereof decreases from its value in air. Such a TGS sensor may typically exhibit a maximum resistance in clean air of 100K ohms which may decrease to as low as 10K ohms in the presence of an appropriate concentration of various gases.
Prior methods of detecting gas concentration using TGS sensors have included detecting a change in current through it. Such sensing techniques have the disadvantage that a sudden change in power supply or reference voltage changes the operating characteristics of the detecting circuit and either produces a false detection or fails to produce a detection when it should.